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Is it possible to connect a pciE slot to one of many Root Complexes, as if the device network is a fully connected graph

Let me give an example.

  • Number of Sockets - 1
  • Number of CPUs per Socket - 2 (Cpu0 & Cpu1)
  • Connect two Gpus Gpu1 and Gpu2

Per my understand each Cpu has its own Root Complex RC. Since the system has two Cpu's I have two root complexes RC1 & RC2.

Questions:

  • Can I connect Gpu1 to RC1 or RC2
  • Can I connect Gpu2 to RC2 or RC1
  • Can I connect both Gpu1 & Gpu2 to RC1
  • Can I connect both Gpu1 & Gpu2 to RC2
saidasa
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  • As far as I know there is only one root complex per socket, not one per CPU. – prl Aug 24 '17 at 07:37
  • Are you wiring a motherboard or using an existing one? – prl Aug 24 '17 at 07:38
  • I am not wiring a motherboard, just a regular user. I had to collect some benchmark data on a system that had multiple pciE slots and multiple CPU's per socket – saidasa Aug 25 '17 at 16:43

1 Answers1

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Each PCIe slot on the motherboard is wired to a specific PCIe port. The only way to control which port a specific GPU is connected to is to physically plug it into the right slot. Depending on what ports are wired to which slots, you may or may not be able to do what you want by putting your GPUs in different slots. Your motherboard documentation may tell you which physical slot is connected to which port.

prl
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  • I am just a software user trying to understand. I am trying to understand at a logical level. Being a non-hardware guy I don't follow what you mean by Cpu Socket or the Chipset. – saidasa Aug 25 '17 at 16:42
  • I edited my answer to hopefully make it more helpful. – prl Aug 26 '17 at 03:48
  • Thanks for the comment. I too have this understanding i.e. a port is connected to a particular cpu and not "**selected**" to be connected to a cpu. On a system with two ports p1 and p2 and two cpu's c1 and c2, per first scheme we know which port is connected to which cpu. Per second scheme we can update the Bios where we can configure which port is connected to which cpu. I would think over time we will move to the second scheme so that we can reconfigure the hardware to serve a different use case. – saidasa Aug 27 '17 at 19:54
  • Ports are not connected to specific CPUs within a single chip. – prl Aug 27 '17 at 21:16
  • I just realized that neither the question nor any of the comments mentioned the type of processor. My answer and comments may only apply to Intel parts, since I don't know much about other types. – prl Aug 27 '17 at 21:20
  • Assume it is a Intel Cpu. I guess I was asking what the traditional practice is - is a pciE slot married to a specific Cpu per wiring or is it configurable via Bios update – saidasa Aug 28 '17 at 22:05